Patent Document 1, for example, discloses a head-up display device as a display device and the disclosed head-up display device includes a control board, a backlight board, and a liquid crystal driving board. The control board of Patent Document 1 is able to control the backlight board and the liquid crystal driving board, and cause a light source unit to emit an image of an instrument as display light.
Patent Document 2, for example, discloses a head-up display device, and the disclosed head-up display device includes a control board and a projector. The control board of Patent Document 2 is able to control the projector and cause a vehicle occupant to visually recognize a virtual image of a video projected on a screen.
Patent Document 3, for example, discloses a display device, and the disclosed display device includes a control unit, an illumination control unit, and a display control unit. The control unit of Patent Document 3 is able to control the illumination control unit and the display control unit, and cause a vehicle occupant to visually recognize a virtual image of light from a screen (a display image).
More specifically, Patent Document 3 discloses a display device that displays an image in, for example, the field sequential format. The disclosed display device includes an illumination device, a light intensity detection unit, an illumination optical system, a display element, a projection optical system, a screen, a planar mirror, a concave mirror, a casing, and a translucent portion. In the display device of Patent Document 3, it is possible to change luminance of the illumination device according to luminance required for a display image to be displayed on the screen. Specifically, in the display device of Patent Document 3, two driving methods, for example, are employed as driving methods of the light source unit of the illumination device, and it is possible to change the luminance of the illumination device by changing a combination of a control value required for a PWM driving method (e.g., a duty cycle) and a control value required for a PAM driving method (e.g., a current value).
In the display device of Patent Document 3, a ratio of a display period in a frame period (a time period in which a display element is able to display a display image on the screen) may be constant (e.g., 50%) or may be determined according to luminance required for the display image (e.g., 50% or 70%).